The recent increase in old-age population or in aged individuals easily getting ill has increased the need for diagnoses and the factors accelerating the ageing of individuals, e.g., diabetes, high blood pressures, heart diseases and the like are clearly enumerated, and countermeasures are required to be taken particularly against circulatory organ diseases which are high in the number of deaths and are easy to become chronic. In respect of such diseases, it is of course necessary to give medical treatments, but specialists' guidance of living conditions base on the blood pressure value as a criterion are desirable and, under such circumstances, it is desired to provide a sphygmomanometer with which the blood pressure values can be measured in a simple mnner and accurately.
A standard clinical method of indirectly measuring blood pressures currently employed is based on an auscultatory method by means of Korotkoff's sound.
In this method, air is fed usually at a rate of 20 to 30 mmHg/sec. into a cuff wound on a subject's upper arm having a large artery in the surface skin portion to give an air pressure to the cuff to be 10 to 20 mmHg higher than an anticipated systolic blood pressure value and to press the large artery and stop blood flow therein. Then the air is exhausted usually at a rate of 2 to 3 mmHg/sec. to gradually reduce the air pressure in the cuff; that is, the cuff pressure, the blood flow generated again is known by Korotkoff's sounds and the cuff pressure at this time is measured as the systolic blood pressure value. Incidentally, the Korotkoff's sounds are attenuated vibration sounds having a frequency higher than that of arterial pulse waves, which are produced in the side direction of blood vessel wall at the time when the large artery is slightly opened due to the gradual reduction of the cuff pressure and thus the blood flow of the same arterial pulse wave as that at the time of systole begins to pass through the blood vessel at a high speed and appears as synchronized with the arterial pulse wave. In this method, the cuff pressure when the Korotkoff's sounds appear is made the systolic blood pressure value. When the exhausting of air out of the cuff is further continued, the timing of the appearance of the Korotkoff's sounds appearing as synchronized with the arterial pulse wave at the time of systole (corresponding to peaks of arterial pulse curve) will shift to the side of diastole (corresponding to valleys of the arterial pulse curve) and, at the moment when the korotkoff's sounds appear as synchronized with the diastole is reached, the large artery is already opened so that the blood flow will be no longer throttled and the Korotkoff's sounds will disappear. The cuff pressure at this time is measured as the diastolic blood pressure value.
There are electronic and mechanical types of such sphygmomanometer provided to thus indirectly measure the systolic and diastolic blood pressure values. In these measuring instruments, the appearance and disappearance of the Korotkoff's sounds are detected with one's auditory sense using a microphone or stethoscope, and the pressure indicating value of a mercury or aneroid sphygmomanometer is traced with eyes to be memorized and measured, while increasing and decreasing the pressure to the cuff. Therefore, such measurement depends on the auditory and visual senses so that a skill is required to a measurer for achieving the accuracy. Even if the skill is high, the measuring precision is solely dependent on the subjective judgment of the measurer as described above, so that no accurate measurement will be expected. Further, there is a type of such measuring instrument for use of medical practitioners and specialists that the pressure is automatically increased and decreased but, as the method of increasing and setting the cuff pressure above the anticipated systolic blood pressure value depends on the subjective judgment, there has been a problem that the measuring precision often fluctuates.